ATLANTA (AP) — Health officials on Thursday reported a five-fold increase of cases of a new strain of swine flu that spreads from pigs to people.

The cumulative case count jumped from 29 a week ago to 158 this week, thanks to a wave of new cases confirmed in Indiana and Ohio, the Centers for Disease Control and Prevention said. Most of the cases have been tied to state and county agricultural fairs, where visitors are put in close contact with infected pigs, said Dr. Joseph Bresee of the CDC.

The recent cases include at least 113 in Indiana, 30 in Ohio, one in Hawaii and one in Illinois, Dr. Bresee said.

Most of the infected patients are children — probably because many were working closely with raising, displaying and visiting pigs at the agricultural fairs, Dr. Bresee said.

Also, diagnosis of cases has become quicker in the past week. CDC no longer must confirm a case with its own lab. Now states are using CDC test kits to confirm cases on their own, speeding the process along. The patients likely were infected a week or two ago.

The CDC has been tracking cases since last summer. A concern: the new strain has a gene from the 2009 pandemic strain that might let it spread more easily than pig viruses normally do.

The good news is the flu does not seem to be unusually dangerous. Almost all of the illnesses have been mild, and no one has died. Two of the recent cases were hospitalized, but both recovered and were discharged, added Dr. Bresee, the agency’s chief of influenza epidemiology.

More good news is that all of the recent cases appear to have spread from pigs to humans, meaning it’s not very contagious, at least between people.

But there probably will be more cases in the weeks ahead, and it won’t be surprising if at least a few of them involve person-to-person transmission, Dr. Bresee said.